Scanning electron beam computed tomography systems are described generally in U.S. Pat. No. 4,352,021 (Boyd, et al.) issued Sep. 28, 1982. The theory and implementation of devices to help control the electron beam in such systems is described in detail in U.S. Pat. No. 4,521,900 (Rand, et al.), issued Jun. 4, 1985; U.S. Pat. No. 4,521,901 (Rand, et al.) issued Jun. 4, 1985; U.S. Pat. No. 4,625,150 (Rand, et al.) issued Nov. 25, 1986; and U.S. Pat. No. 4,644,168 (Rand, et al.) issued Feb. 17, 1987. Applicants refer to and incorporate herein by reference each above listed patent to Rand, et al.
As described in detail in U.S. Pat. No. 4,521,900 to Rand, et al. (hereafter "Rand, et al. '900"), an electron beam is produced by an electron gun at the upstream end of an evacuated generally elongated and conical shaped housing chamber (or "drift tube"). A large electron gun potential (e.g., 130 kV) accelerates the electron beam downstream along a first straight line path defining the chamber Z-axis. Further downstream a beam optical system including focus and deflection coils deflects the beam into a scanning path. The deflected beam exits the beam optical system and impinges a suitable target for producing X-rays. The X-rays penetrate an object (e.g., a person) and are then detected and computer processed to produce an X-ray image of a portion of the object. Prior art electron beam systems such as described in the above-referenced patents characteristically had relatively long conical shaped housing chambers, e.g., 3.8 meters.
Because the electrons are negatively charged, the resultant space-charge causes the electron beam to diverge or expand in the upstream chamber region between the electron gun and the focus and deflection coils. This expansion is beneficial because the beam diameter at the target varies approximately inversely with the beam diameter at the focus and deflection coils. In the chamber region downstream from the focus and deflection coils, a converging electron beam is desired. In that downstream region, the beam preferably is neutralized by positive ions produced by the electrons from residual gas in the chamber, or from a gas purposely introduced into the chamber. This neutralization causes the beam to self-focus sharply upon the target to produce a sharp X-ray image. In the ideal case, the electron beam is perfectly uniform in current density, diverging upstream and converging to sharply self-focus downstream.
Although a diverging beam is desired in the upstream chamber region, positive ions can counteract divergence. Positive ions are present because the electron beam interacts with residual gases that inevitably remain after evacuation, or with gases purposely introduced into the chamber. In the upstream chamber region, positive ions are detrimental because they tend to neutralize the space-charge, preventing electron beam divergence. This in turn increases the beam width at the target, resulting in a defocused X-ray image. Neutralization also can result in the beam becoming unstable and collapsing completely.
By contrast, positive ion neutralization can be beneficial in the chamber region downstream from the focus-deflection coils. Here neutralization eliminates the electron self-repulsion, while the beam's attractive magnetic field converges and self-focuses the beam. Elements of the beam optical system are then used to fine tune the converged beam to produce a sharp X-ray image.
Thus, while positive ions can be beneficial downstream from the focus-deflection coils, they are detrimental in the upstream region. In prior art tomography systems such as described in the Rand, et al. U.S. Pat. No. 4,521,900 patent, positive ions were removed by causing the electron beam to pass axially through an electrically biased ion clearing electrode (or "ICE") mounted in the upstream chamber region. The ICE created a relatively large transverse electric field that swept away the slow moving positive ions, without disturbing the considerably faster moving electrons. Such ICEs required large electrode potentials (e.g., about 1 kV) to produce the large electric field needed to remove ions on an "all or nothing" basis.
Ideally the electron beam should be homogenous, i.e., with a uniform electron distribution, so the beam acts as its own perfect lens: self-diverging in the upstream chamber region and self-converging in the downstream chamber region to focus sharply on the target. A uniform space-charge density is desired because any optical aberrations due to the electron beam self-forces would then be eliminated. In addition to degradation from ions, the electron beam space-charge density may not be perfectly uniform due to imperfections in the electron gun and in the beam optics system.
It is believed that the relatively long length of prior art housing chambers contributed to beam space-charge homogenization by smoothing or evening out the electron distribution. In essence, the distance between the electron gun and beam optics was sufficiently long to allow the electron beam to expand and become more uniform without requiring special mechanisms to compensate for beam non-uniformity.
For reasons of economy, maintenance and ease of installation in hospitals, it is advantageous to construct a scanning electron beam system using a housing chamber shorter than used in prior art systems. Unfortunately, however, the resultant shorter distance between the electron gun and beam optics prevents the beam from expanding sufficiently to become homogeneous. Further, the construction of shorter housing chambers may create discontinuities, typically near vacuum valve couplings and flanges. These discontinuities create gaps in the electric field generated by ion controlling devices, thus allowing some ions to remain in the upstream region where they further degrade beam expansion.
In summary, in an electron beam scanner system employing a relatively short length housing chamber, there is a need for a method and apparatus for removing positive ions, and for controlling the positive ion distribution. Such method and apparatus should compensate for beam space-charge density non-uniformity, thereby eliminating any aberrations due to the beam self-forces. Unfortunately, prior art ICEs with their "all or nothing" characteristic simply do not provide any mechanism for controlling space-charge uniformity of the electron beam, and do not remove all ions when operating over discontinuities. The present invention discloses an ion controlling electrode assembly and a method to fulfill these needs.